A 2010 Chevrolet Malibu smashes into the back of a 2007 Hyundai Translead semitrailer in a crash test. / IIHS

by Larry Copeland, USA TODAY

by Larry Copeland, USA TODAY

Most big-rig manufacturers are still building tractor-trailers with rear under-ride guards that allow cars going as slow as 35 mph to slide under the trucks in crashes, according to new research from the Insurance Institute for Highway Safety.

Under-ride guards, those metal bars hanging down from the backs of tractor-trailers, are meant to prevent horrific crashes that can shear the top off a car and behead its occupants.

Two years ago, the Insurance Institute released research showing that many of the guards were ineffective. It found that the federal government's minimum strength and dimension requirements for under-ride guards were inadequate. The IIHS petitioned the National Highway Traffic Safety Administration to impose tougher standards.

The NHTSA has not changed its standards, but trailer manufacturers are making stronger guards than the government requires, partly because since 2007, Canada has had a much tougher under-ride standard than the USA.

The Insurance Institute says that under-ride guards still don't prevent all under-ride crashes. IIHS researchers tested under-ride guards from the nation's eight largest trailer manufacturers, representing about 80% of all manufacturers, said David Zuby, the institute's chief research officer.

The NHTSA "is actively working to improve truck under-ride protection and recently completed an in-depth field analysis â?¦ to support potential changes to existing federal safety standards," said NHTSA administrator David Strickland. "The driving public should know that we are actively working to address the issues raised in IIHS's report and that their safety will always be our top priority."

The agency recently completed an in-depth field analysis of two years' worth of fatal under-ride crashes with the University of Michigan Transportation Research Institute.

In a series of three progressively tougher crash tests, IIHS engineers crashed a 2010 Chevrolet Malibu going 35 mph into a parked truck. The trucks were equipped with guards that met both the U.S. and Canadian standards.

All eight guards prevented under-ride in the first scenario, which was a dead-on, rear end collision.

In the second test, in which only half the width of the car overlapped with the rear end of the trailer, all but one trailer passed.

When the overlap was reduced to 30% - when only about 30% of the Malibu's front end struck the trailer - every trailer except one failed, the Insurance Institute said.

The one truck that passed all three tests: a trailer from Canadian manufacturer Manac, which sells trailers in the USA under the name Trailmobile.

"All eight trailers meet the (tougher) Canadian standard," Zuby said. "Yet seven of them failed to prevent under-ride in the narrow overlap category."

He said the Manac guard "is not made of different metal or heavier material than the others." The company reinforced the attachment points for its guard and mounted the vertical supports closer to the edges of the trailer, Zuby said. Insurance Institute spokesman Russ Rader said those modifications cost about $20 per trailer.

In 2011, 260 of the 2,241 passenger vehicle occupants killed in large truck crashes died in under-ride crashes, down from 460 of 3,693 in 2004, the IIHS said, adding that the decline is partly due to the recession-induced drop in miles traveled.

"This report shows that since IIHS' first study in 2011, trailer manufacturers have responded favorably and have already taken steps to surpass federal standards for these guards," said Sean McNally, spokesman for the American Trucking Associations, which represents the nation's largest trucking companies.

"We must also focus on preventing these kinds of crashes in the first place," he said. "More driver education on sharing the road with large commercial vehicles is a must, and promoting greater use of collision avoidance technology in both cars and trucks will also produce results."